Thermotransfer copy process wherein a heat sink is positioned within the composite



Feb. 18, 1964 R. B. RUSSELL 3,

THERMOTRANSFER COPY PROCESS WHEREIN A HEAT SINK IS POSITIONED WITHIN THECOMPOSITE Filed June 21, 1960 PRESSURE BLADE WAX TRANSFER SHEET PLASTICFILM IN VEN TOR.

M fl M I am m-AWM ATTORNEYS United States Patent 3,121,791THERWOTRANSFER GJGPY PROES WHEREHN A HEAT SHNK If; POSITIONED WITHIN THEtZfiMPGSlTE Robert B. Russell, 248 Park 5%., Newton, Mass. Filed June21, 196i), Ser. No. 37,669 2 (Ilaims. (Cl. 250-65) This inventionrelates to thermographic copying and more particularly to improvementswhich enhance definition and eliminate background when employed with thethermographic printing process described in my prior and copendingapplication Serial No. 821,943, filed June 22, 1959 now Patent No.3,048,695.

A brief discussion of certain background facts will best lead to anunderstanding of the invention. While making copies by the machine andprocess of my above said copending application, I have noticed variouscurious things for which there has hitherto been no apparentexplanation. For instance, a crease in the original results in atransfer of wax from the transfer sheet to the copy sheet along the lineof the crease. Several authorities have attributed this effect to thescattering of the incident radiation by the walls of the crease and/orto small amounts of dirt collected in the crease. However, various factsshow that these explanations are not actually complete. Thus the sameform of unwanted wax transfer occurs at the end or sides of the originalwhere the wax melts and transfers markedly along the line of the edge ofthe sheet. along these edges to explain such increased heat. Again,while printing raised letters as for instance letters printed byengraving, the melting and transfer of the wax extends radically to eachside of the letter body and far beyond the probable extent of heatemanating directly from the black portion of the letter.

In searching for an explanation of these curiosities, I have found theyare due at least to a substantial degree to the insulative power of air.Whenever there is an irregular air gap between the copy and theoriginal, the wax over the air gap appears to melt and transfer atnearly the same rate as it does over the black letters on the original.To test this, I have made copy by my process employing an original thathas letters deeply imprinted on its surface but without black ink in theletters. These non-heat absorbing letters produce perfectly legiblecopy. The apparent explanation is that, as the incident radiation isdirected through the wax and copy sheets to the original, each sheet isabsorbing heat and its temperature is rapidly rising. The original,however, being the most protected part remains relatively cool comparedto the copy. Thus heat commences flowing toward the original. As theletters on the original become hotter than the copy sheet due to thegreater absorption of the incident radiation which penetrates to theoriginal, the path of heat from the copy toward the original at thoseplaces is blocked. This causes the copy to get hotter and melt the waxselectively over the letters. But in a like manner, if the heat path isblocked by the insulative power of an air gap, a similar result takesplace and the wax is caused to melt whether or not a letter is there.

The insulative power of air also affects the problem in another way.Thus when an original of irregular density is employed, the portions ofthe letters overlying the less dense portions of the original becomehotter and cause more pronounced printing in those areas. The reason forthis is that the dissipation of heat by conduction into the origintal isless in those areas. A third way by which it affects the problem has todo with the nature of the surface to which the incident radiation isapplied. Thus while a smooth-walled air gap in the original will printas indicated above, a jagged or fibrous air gap prints No scattering oflight or dirt is present "ice to a far greater degree. In this case thesmall, free fibers are less able to lose their heat by conduction intothe original.

These discoveries have led to the present invention and have made itpossible for me substantially to improve the quality of printingdifficult originals with the machine and process or" my above said priorapplication.

Recognizing that air gaps should be eliminated is not of itself anadvance in the art. This is because good confact with the letters on theface of the original has always been an object of the prior workers inthe field who have uniformly based their processes on the theory thatthe heat is transferred from the black, infra-red absorbing letters tothe copy by conduction alone. Thus smoothness and elimination of airgaps have always been obvious 0bjectives in the prior processes. With myinvention, however, I am not so interested in good contact with theletters as I am in eliminating non-uniform air gaps between the copy andthe white background areas of the original. In my process, the provisionof a good point to point contact between the letters and the copy sheetis actually not necessary for two reasons; first, an air gap over theletters can actually increase the selective melting; and second, in myprocess, that portion of the return heat which is attributable toradiation is not adversely affected by the air space. On the other hand,in my process, uniform contact is important in the background areas ofthe original because there must be a uniform flow of heat from the (WZlXand copy sheets toward the original in those areas and without suchcontact the flow will be localized and unwanted printing will occur inthe background areas.

Accordingly, in the practice of my invention herein described, I takepains to provide a smooth wax transfer sheet and a smooth uniformlydense copy sheet. These are in part features of my invention as willbecome apparent from the detailed description which follows. Smoothingand rendering these sheets uniformly dense alone, however, only producesmaximum results when the original is also smooth and uniformly dense.Thus I can produce excellent copy with smooth wax and copy sheets on afinely printed smooth original, but I have difficulty achieving goodresults when copying newsprint with the same wax and copy sheets. Aspreviously mentioned, an engraved image prints poorly by my processbecause its letters are raised leaving air gaps adjacent to the bodiesof the letters.

This invention, therefore, has additionally as a major object, theelimination of the harmful effects of surface roughness and unevendensity in the original.

I do this in a preferred embodiment of my invention herein described byplacing a thin film of clear plastic between the original and the copy.The effect of this is to provide a relatively uniform heat sink betweenthe copy and the original so that heat flow towards the original fromthe wax and copy sheets is substantially smoothed out. Of course, airgaps between this film and the original can still render the flow unevento some extent, but the film serves to reduce their harmful effect. Onthe other hand, the film absorbs but little of the incident radiationand permits the black letters on the original freely to heat up morethan the background. Thereafter since the area of the letters isrelatively large compared to the thickness of the film, and since thefilrn is both a good conductor and a good radiation transmitter, thefilm does not substantially interfere with the process of selectivelymelting the wax over the letters. The result is greatly to improve thecopying quality of non-uniform originals such as newsprint.

Other objects and features will best be understood with reference to thefollowing description and drawing in which:

The single figure is a diagrammatic representation of the lay-up ofsheets employed in one embodiment of my invention.

In the preferred embodiment of my invention herein shown, I employ thegeneral process described in my above said copending application. Thisinvolves the use of a thin, wax-coated transfer sheet 18 having arelatively non-infra-red absorbent dye in the wax. This transfer sheet10 is placed over a copy sheet 12;, and the copy sheet is then placedover an original 14 hearing letters or other indicia to be copied.Copying is carried out by directing a brief and intense blast ofirradiation through the wax transfer and copy sheets to the surface ofthe original where the letters are heated to a higher temperature thanthe background areas by selective absorption of the incident radiation.In my process, the wax transfor sheet also absorbs some of the incidentradiation, as do the copy sheets and background areas of the original.But since the wax transfer and copy sheets are outermost, theirtemperatures rise more rapidly. This causes a flow of heat inwardly fromthe wax transfer and copy sheets toward the original. In the areas wherethe heated letters are located, however, or in the places where thereare air gaps, this inward flow of heat is blocked, and the copy sheettemperature accordingly rises more rapidly in those places. Of courses,when the letters become sufficiently hot, heat emanates from thembackwardly toward the wax and a portion of this return heat is in theform of radiation.

In order to eliminate unevenness of the inward flow of heat from thecopy sheet 12 toward the original 14, I take several specialprecautions. First, I make the wax sheet 10 as thin as possible andprovide it with an ample wax coating. I find that the wax itselftransmits both the heat and radiation relatively rapidly and uniformly,but that the fibers in the paper backer for the wax transfer sheet are adefinite source of uneven temperature rise and heat fiow. In my abovesaid application, I mentioned employing a 9 lb. per ream paper (3300square feet) called "Trent. I find now I can use a /2 lb. per ream Trentto even better advantage.

The copy sheet 12 need not be especially thin, as long as it is highlyuniform both in infra-red absorption and heat conduction. (Of course, itcannot be infra-red opaque.) Thus I find I can make excellent copy on a34 lb. coated paper in which a conventional mineral filler and caseincoating is applied very flatly and with uniform density as by bladecoating. Preferably the paper should be coated on both sides.

In the preferred embodiment of this invention, I place a .001 film ofMylar 16 between the original 14 and the copy 12. Of course, the Mylar16 is a good transmitter of the incident radiation, so it interposes butlittle hindrance to the selective heating of the letters on theoriginal. On the other hand, the Mylar is very smooth, has a relativelyhigh specific heat as compared to paper, and conducts heat very rapidlyas compared to air. Thus the flow of heat to the Mylar is substantiallyuniform and remains so until the return heat from the letters issufficient to melt the wax. In effect, the Mylar is a buffer, or damper,eliminating the effect of small disconformities in the original butpermitting major heat variations to be felt by the wax sheet 16. Inorder to facilitate po oning and repeated use, I mount the Mylar film 16on a backer 18, and place the original between the two. In my ma chineas in the drawing herein, these pieces go in face down and issue face upafter passing under the light.

It should be noted that my Mylar sheet 16 is between the original andthe copy. I emphasize this because I am aware of a competitive processwhich places the original and a copy sheet under a plastic flap mountedon a hacker. The similarity is only superficial because in the saidcompetitive process, the plastic flap is definitely not placed betweenthe copy and the original. In fact, by that process, as by any othertherniographic processes of which I am aware, placing anything betweenthe original and the copy is emphatically discouraged.

Mylar has some disadvantage from the viewpoint of aging in that itbuckles slightly under the influence of heat. Cellulose acetate doeslikewise. Of course this disadvantage does not prevent these materialsfrom being operative in the context. Nylon and polyethylene areadequate. In fact, any clear, flexible plastic which is smooth andrelatively non'absorbent to radiation of a wave length between about .5to 3 microns, will do, and accordingly it is not my intention to limitthis particular embodiment of my invention to the precise form ofplastic shown herein, but rather to claim it broadly as to the class ofsmooth, clear, flexible and light transmitting plastics.

Also in view of the teachings of this invention, a number ofmodifications or variations will be apparent to those skilled in the artand therefore, it is also my object to include them within the scope ofthe claims. Recognizing that the evening or damping effect of the film16 is due to the fact that it is cooler than the original and copy sheetbetween which it lies (because it absorbs less of the incidentradiation) it is apparent that the inner side of the copy sheet itselfcan be treated to approximate this same function. Thus a thinapplication of cellulose acetate or polyethylene on the surface of thecopy adjacent to the original improves results. Conversely a coating ofwhite mineral coating materials on the surface of a copy sheet made ofclear film, with the said coating being placed adjacent to the wax,provides good results. The important thing is to provide a temperaturegradient in the copy from warmer, adjacent to the wax, to cooleradjacent to the original and back again to warmer in the original. Inthis way the part immediately adjacent to the original serves somewhatas a heat sink and thereby mask the tendency of uneven flow of heat intothe original without preventing an intense build-up of heat in theletters on the original from being felt through the copy and back to theWax.

Of course, cost is an important factor and although such complexcombinations of plastic coated papers or coated films provide excellentcopy, they are expensive. A good workable compromise is provided byapplying a coating of wax to a thin paper sheet. The trouble with a waxcoating on the copy, however, is that it may melt and deposit on theoriginal. For this reason high melting point waxes are chosen for thisparticular purpose.

The possibility of harming the original is ever present in the practiceof my invention and is one of the reasons why I prefer the embodiment ofmy invention which employs the thin flap of Mylar 16 herein specificallydescribed. It serves to protect the original in cases of inadvertentmishandling of the machine.

A second embodiment of this invention has a special application in thereproduction of signatures written with ball point pens. This is a topicwhich has in the past initiated considerable criticism of infra-redthermographic copying processes. By re-arranging certain of thecomponents described above and by heavily indenting the impression ofthe point of the pens, such signatures can be copied by my process.Thus, a smooth uniformly dense relatively transparent original isemployed. The copy sheet is preferably of greater infra-red absorbencythan the original, and is also smooth and uniformly dense. Preferably athin, smooth wax transfer sheet is used. With this combination, anindentation on the face of the original causes an air gap which in turnblocks the flow of heat from the copy toward the original. This resultsin a build-up of heat in the copy which selectively melts the wax andcauses it to print where the indentation is. In this instance tnesmooth, uniformly adsorbent and relatively cool heat sink is in theoriginal itself, and it performs the evening or damping functiondescribed above except in places where the letters or intentionalindentations appear on its face. In this way the ball point" ink is notthe cause of the selective melting of the wax but rather the indentationcaused by the pen point.

By the foregoing teachings it will now be clear to those skilled in theart that various adjustments in the infrared absorbency of thecomponents will be dictated by the type of copying involved. If voids orindentations alone are being copied a relatively sharp gradient is bestwith the copy being both comparatively highly conductive and highlyinfra-red absorbent, and with the original being highly conductive. Inthis way the copy gains heat more rapidly and heat flows from it bothtoward the wax and toward the original. In this case, the original isrelatively cool because but little incident radiation reaches it. Thevoids in this case rather than the black letters are then the majorblock to the passage of heat into the original and by causing the heatto build up locally in the copy, they in turn cause the wax to melt andtransfer from the wax sheet to the copy sheet in those areas. With thisarrangement, however, the copy must not be so thick or so absorbent toinfra-red that the temperature of its surface adjacent to the wax risestoo rapidly in relation to the temperature of its opposite surfaceadjacent to the original. Thus photographic film with a black coating onone side makes excellent copy from voids in an original if the blackcoated side is placed adjacent to the voids, but virtually no copy atall if the black coated side is placed adjacent to the wax.

By increasing the infra-red transparency of the copy, the heat flowinterference from the black letters on the face of the original can bemade to be about equal to that of the voids so that they will print withabout equal intensity. Of course, when the copy is highly transparent tothe incident infra-red it remains relatively cool, heat flows toward itrather than away from it, and the hot letters on the original causeprinting more readily than do the voids. This latter arrangement isillustrated by the above described embodiment in which a Mylar flap isinterposed between the original and the copy.

It will be noted, however, that the common point between the foregoingarrangements is that in each one a smooth, uniformly dense, andrelatively good conductor is employed in the manifolded sheets in such away that it is cooler than its surroundings and serves as a heat sinkwhich permits uniform temperature rise of the copy and wax transfersheets and a more accurate control of selective wax melting to producecopy.

Having thus described and disclosed preferred embodiments of myinvention, what I claim as new and desire to secure by Letters Patent ofthe United States is:

1. In a thermally induced transfer copying process in which a copy sheetis placed over an original having indicia thereon to be copied and atransfer sheet having a heat meltable dyed coating thereon is placedover the copy sheet with the said coating adjacent to said copy sheet,the improvement comprising: providing said copy sheet with asubstantially smooth, uniformly dense, transparent and conductive heatsink which is substantially as transmissive to radiation as said copysheet between said copy and said original; thereafter applying intense,richly infrared irradiation to said original through said transfer andcopy sheets and through said heat sink, whereby the temperature of saidtransfer sheet is caused to rise more rapidly than that of said copysheet and said heat sink respectively thereby causing a flow of heattherefrom toward said heat sink; said heat sink being more transparentthan said original and selected in relation thereto to transmitsufiioient incident radiation to said original to cause said original torise in temperature due to said incident irradiation at about the samerate as said heat sink; whereby voids on the surface of said originalhave no substantial influence in the heat flow pattern between saidadjacent members.

2. In a thermally induced transfer copying process in which a copy sheetis placed over an original having indicia thereon to be copied and atransfer sheet having a heat meltable dyed coating thereon is placedover the copy sheet with the said coating adjacent to said copy sheet,the improvement comprising: placing next to said copy sheet a separate,substantially smooth, uniformly dense, transparent and conductive heatsink which is substantially as transmissive to radiation as said copysheet between said copy and said original; thereafter applying intense,richly infra-red irradiation to said original through said transfer andcopy sheets and through said heat sink, whereby the temperature of saidtransfer sheet is caused to rise more rapidly than that of said copysheet and said heat sink respectively thereby causing a flow of heattherefrom toward said heat sink; said heat sink being more transparentthan said original and selected in relation thereto to transmitsutficient incident radiation to said original to cause said original torise in temperature due to said incident irradiation at about the samerate as said heat sink; whereby voids on the surface of said originalhave no substantial influence in the heat flow pattern between saidadjacent members.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A THERMALLY INDUCED TRANSFER COPYING PROCESS IN WHICH A COPY SHEETIS PLACED OVER AN ORIGINAL HAVING INDICIA THEREON TO BE COPIED AND ATRANSFER SHEET HAVING A HEAT MELTABLE DYED COATING THEREON IS PLACEDOVER THE COPY SHEET WITH THE SAID COATING ADJACENT TO SAID COPY SHEET,THE IMPROVEMENT COMPRISING: PROVIDING SAID COPY SHEET WITH ASUBSTANTIALLY SMOOTH, UNIFORMLY DENSE, TRANSPARENT AND CONDUCTIVE HEATSINK WHICH IS SUBSTANTIALLY AS TRANSMISSIVE TO RADIATION AS SAID COPYSHEET BETWEEN SAID COPY AND SAID ORIGINAL; THEREAFTER APPLYING INTENSE,RICHLY INFRA-RED IRRADIATION TO SAID ORIGINAL THROUGH SAID TRANSFER ANDCOPY SHEETS AND THROUGH SAID HEAT SINK, WHEREBY THE TEMPERATURE OF SAIDTRANSFER SHEET IS CAUSED TO RISE MORE RAPIDLY THAN THAT OF SAID COPYSHEET AND SAID HEAT SINK RESPECTIVELY THEREBY CAUSING A FLOW OF HEATTHEREFROM TOWARD SAID HEAT SINK; SAID HEAT SINK BEING MORE TRANSPARENTTHAN SAID ORIGINAL AND SELECTED IN RELATION THERETO TO TRANSMITSUFFICIENT INCIDENT RADIATION TO SAID ORIGINAL TO CAUSE SAID ORIGINAL TORISE IN TEMPERATURE DUE TO SAID INCIDENT IRRADIATION AT ABOUT THE SAMERATE AS SAID HEAT SINK; WHEREBY VOIDS ON THE SURFACE OF SAID ORIGINALHAVE NO SUBSTANTIAL INFLUENCE IN THE HEAT FLOW PATTERN BETWEEN SAIDADJACENT MEMBERS.